I don't like the idea of mixing aileron. It's really just covering up an existing problem. That solution also adds unnecessary drag.

Face the front of your airplane so that the elevators are just hidden behind the wings. Now slowly pull back on the elevator. Does one half appear before the other? If so, that's the high half. The halves may also look straight when neutral, but misalign after being moved because of asymmetric travel. This is usually caused by one of the servo horns not quite being set up square at neutral (perpendicular).